In recent years, studless tires are widely used instead of spiked tires because the spiked tires cause damage to the surface of paved roads and dust pollution.
The studless tires have been remarkably improved in on-the-snow running performance as a result of employing a block type tread pattern comprising a large number of blocks provided with sipes and a tread rubber material being capable of keeping suppleness under very low temperature conditions.
Such studless tires are, however, still inferior to the spiked tires in running performance on iced roads, especially the so called mirrorbahn--an iced road which is very smooth like a mirror. Therefore, a pressing need is to improve running performance on mirrorbahn.
In order to improve on-the-ice running performance, it is important not only to increase the critical grip of the tire but also to decrease the difference between the critical grip and the remaining grip when exceeding the critical grip, namely the drop of grip.
In general, conventional block type tread patterns for passenger car tires comprise circumferential rows of blocks which are substantially the same size and same shape. In this case, it is easy to increase the critical grip at the sacrifice of wear resistance, but the drop of grip becomes large. In such a tire, it is difficult to control the vehicle during an emergency. On the other hand, tires whose grip drop is small are usually low in the critical grip by nature. Thus, a high critical grip and small drop of grip are contradictory requirements.
It is therefore an object of the present invention to provide a pneumatic tire in which the critical grip is increased and at the same time the drop of grip is decreased to improve on-the-ice running performance without decreasing the resistance to uneven wear.